Keyed stereophonic transmission system

ABSTRACT

A stereophonic transmission system having two sources of audio signals in which the audio signal of an ultrasonic frequency is keyed in opposite phase for periods of substantially one-half cycle of said frequency to derive the respective trains of pulses amplitude-modulated by said audio signals and displaced in phase relative to each other by the width of the pulses. The trains of pulses are super-imposed and admitted simultaneously to the input of the transmission channel and subsequently separated into the pulse trains at the output of the transmission channel.

United States Patent Svorec [45] July 25, 1972 s41 KEYED STEREOPHONIC2,792,449 5/1957 30mm ..179/15 TRANSNIISSION SYSTEM 2,336,276 12/1943Van Der Meulen ...1789/l5 BT 2,921,981 1/1960 Kidd ..l79/l5 B1 [721Invent Rich 324 36th St, South 3,306,981 2/1967 l-lecht ..179/1s STBend, Ind. 46615 22 Filed; June 19 19 9 Primary Examiner-Kathleen H.Claffy Assistant Examiner-Tom D'Amico I 1 Appl- 834,748Attorney-Marmaduke A. Hobbs and Kemon, Palmer,

Related U.S. Application Data Stewart Estabrook [63] Continuation ofSer. No. 388,979, Aug. 8, 1964, [57] ABSTRACT abandoned A stereophonictransmission system having two sources of 52] U S Cl 179/15 BT audiosignals in which the audio signal of an ultrasonic [5]] Km Huh 5/00frequency is keyed in opposite phase for periods of substam 58] FieldSearch 5 HT 1 325/36 15 BP tially one-half cycle of said frequency toderive the respective 325/15 1 5 trains of pulses amplitude-modulated bysaid audio signals and displaced in phase relative to each other by thewidth of the pulses. The trains of pulses are super-imposed and admitted[56] References Cited simultaneously to the input of the transmissionchannel and UNITED STATES PATENTS subsequently separated into the pulsetrains at the output of the transmission channel. 3,018,335 1/1962 DeRosa ..l79/1G 2,007,809 7/1935 Nicolson 179/15 A 2 Claims, 10 DrawingFigures 7 jl/TFl/T KEYED STEREOPHONIC TRANSMISSION SYSTEM Thisapplication is a continuation of my copending application Ser. No.388,979 filed August 8, 1964 now abandoned.

A main object of the invention is to provide a novel and improvedapparatus for utilizing a single amplifier or transmission channel forhandling two stereophonically related signals the apparatus involvingsimple components, being reliable in operation, and providing asubstantial saving in cost as over previously used arrangements whereinseparate amplifiers or transmission channels are employed for therespective stereophonically related signals.

A further object of the invention is to provide an improved means fortransmitting a pair of stereophonically related signals through a singletransmission channel, the means being inexpensive to fabricate,involving relatively simple and durable parts, and having very low powerrequirements.

A still further object of the invention is to provide an improvedapparatus for utilizing a single amplifier or transmission channel fortransmitting a pair of stereophonically related audio signals, theapparatus being adaptable for use in conjunction with a stereophonicphonograph pickup assembly, or similar stereophonic transducer, andbeing arranged so that the derived stereophonically related signals willbe amplified without substantial distortion and then will be separatelyreproduced so as to reproduce the original stereophonic relationship ofthe input signals.

A still further object of the invention is to provide an improvedapparatus for amplifying or transmitting a pair of stereophonicallyrelated signals, the apparatus requiring only a single amplifier, beingarranged so that the amplifier will operate with the same fidelity toprovide the required gain of both of the input signals, no balancingbeing necessary, and the apparatus being arranged so that the amplifiedsignals are properly divided and reproduced in separate reproducingdevices, such as loud speakers, or the like, in the same stereophonicrelationship as the signals originally possessed when introduced intothe system.

Further objects and advantages of the invention will become apparentfrom the following description and claims, and from the accompanyingdrawings, wherein:

FIG. 1 is a block diagram showing a typical apparatus according to thepresent invention.

FIG. 2 is a schematic wiring diagram showing the electrical componentsemployed in the system illustrated in FIG. 1.

FIG. 3 is a block diagram showing a modified monaural to stereophonicamplifier conversion system according to the present invention, theelements shown being those required to convert a pair ofstereophonically related input signals to a composite single-channelsignal suitable for recording or transmission by any conventional means.

FIG. 4 is a block diagram of a reconversion and reproducing systemadapted to reconvert the composite signal derived in FIG. 3 and toreestablish the stereophonically related signals originally introducedinto the system of FIG. 3.

FIG. 5 is a block diagram of a modified system, similar to FIG. 4, butbeing arranged so that the stereophonically related signals will bereproduced regardless of the inability of the transmission medium totransmit higher audio frequencies.

FIG. 6 is a wiring diagram showing the electrical components of thesystem illustrated in FIG. 3.

FIG. 7 is a schematic wiring diagram showing the electrical componentsof the system illustrated in FIG. 4.

FIG. 8 is a schematic wiring diagram showing the electrical componentsof the system illustrated in FIG. 5.

FIG. 9 is a diagram showing the respective keyed signals derived by theoppositely phased keying transistors employed at the input side of aconversion system according to the present invention.

FIG. 10 is a diagram illustrating a substantially equivalent electricalcircuit for a transistor employed as a variable impedance device as inthe system of the present invention.

A prime purpose of the present invention is to provide a system forutilizing a single transmission channel, such as an amplifier, or anyother conventional transmission channel, to

convey a pair of audio signals, for example, a pair of stereophonicallyrelated audio signals, and to subsequently separate the respectivesignal components from the output of the transmission channel and toemploy the separated signals to energize respective reproducing devices,such as loud speakers, or the like, to reproduce the originalstereophonically related signals. The method of the present inventioninvolves keying the respective input audio signals at an ultrasonicfrequency but in opposite phase and for periods of substantiallyone-half cycle or less of said frequency, whereby to derive respectivetrains of pulses which are amplitudemodulated by the input audio signalsand which are displaced in phase relative to each other by one-halfcycle. The respective trains of pulses are then superimposed andsimultaneously admitted into the input of the transmission channel. Theoutput signal leaving the transmission channel is then keyed at the samefrequency and in phase with the keying of the input audio signals,whereby to separate the output of the transmission channel intorespective trains of pulses amplitude-modulated by the original inputsignals and keyed at an ultrasonic frequency, so that the separatedsignals may operate associated reproducing devices to reproduce theoriginal pair of input audio signals.

Referring to the drawings, and more particularly to FIGS. 1 and 2, atypical apparatus for carrying out the method of the present inventionmay comprise a pair of sources of audio signals 11 and 12, asingle-channel amplifier 13, and a pair of sound reproducing circuits 14and 15. The amplifier 13 is provided with a signal input terminal 16 anda signal output terminal 17. The audio source 11 is connected to theamplifier input terminal 16 through a first keying device 18, and theaudio source 12 is connected to the amplifier input terminal 16 througha second keying device 19. As will be presently described, the devices18 and 19 are keyed at an ultrasonic frequency but in opposite phase,whereby to supply trains of pulses to the amplifier input terminal 16which are amplitudemodulated by the respective sources 11 and 12 andwhich are superimposed on each other to define a composite signalsuitable for amplification by the amplifier. Thus, as shown in FIG. 9, afirst train of pulses 20 appears at the output of the keyer device 18and a second train of pulses 21 appears at the output of the keyer 19.Since the pulses 20 and 21 are displaced in phase by degrees, therespective trains of pulses 20 and 21 may be combined to derive acomposite single audio signal. Similarly, the signal leaving theamplifier 13 at the output terminal 17 thereof may be keyed in the samemanner to separate the components of the composite signal and to deriverespective trains of amplified pulses 20 and 21 which may be separatelyreproduced in the respective reproducing circuits 14 and 15. As shown inFIG. 1, the output terminal 17 of the amplifier 13 is connected to therespective reproducing circuits l4 and 15 through the respective keyingdevices 22 and 23 which are similar to the keying devices 18 and 19 andwhich are respectively operated in phase with said keying devices 18 and19.

In the arrangement illustrated in FIGS. 1 and 2, a common ultrasonicoscillator 24 is employed to simultaneously operate the keying devices18, 19 and 22, 23 at a common ultrasonic frequency and to maintain 180degrees phase displacement between the keyers 18 and 19 and the keyers22 and 23, and

to maintain the keyer 18 in phase with keyer 22 and keyer 19 in phasewith keyer 23.

Referring now to FIG. 2, it will be seen that the respective sources 11and 12 may comprise the respective transducing coils of a conventionalstereophonic phonograph pickup of the magnetic type. The transducingdevices 11 and 12 have a common ground connection 25 which may beconnected to the cathode or the ground terminal of the amplifier 13, ina conventional manner. The ungrounded terminals of the transducer coils11 and 12 are connected to the input terminal 16 of the amplifier 13through respective transistors 26 and 27, which are employed as keyingdevices in a manner presently to be described. The output terminal 17 ofthe amplifier 13 is connected through respective additional transistors28 and 29 to respective sound reproducing circuits including loudspeakers 30 and 31. Thus, the amplifier 13 may include a conventionaloutput transformer 32 having one terminal 33 of its secondary windingconnected to one terminal of each of the loud speakers 30 and 31. Theremaining terminal of the secondary of the transformer 32 constitutesthe output terminal 17, which is connected to the remaining terminal ofeach of the loud speakers 30 and 31 through the transistors 28 and 29.

The ultrasonic oscillator 24 is of conventional construction and employsa transistor 34. The oscillator 24 comprises a transformer 35 having afirst winding 36 and respective additional windings 37, 38 and 39inductively coupled to the first winding 36. A battery 37 has itsnegative terminal connected to the collector 40 of transistor 34 throughthe transformer winding 36. The positive terminal of battery 37 isconnected through a control switch 41 and the transformer winding 37 tothe emitter 42 of transistor 34. The base 43 of the transistor isconnected through a resistance 44 to the wire 45, which is connected tothe positive terminal of battery 37 through switch 41, as shown.Resistance 44 may be of any suitable value, for example, a value ofbetween 50,000 and 100,000 ohms. A condenser 46 is connected across theresistor 44, the capacity of the condenser being of the proper value,taken with the inductance of winding 36 and the mutual inductancethereof with respect to the additionAl windings of transformer 35, toprovide the desired ultrasonic frequency for oscillator 24, namely, afrequency of the order of 20,000 cycles.

As will be readily apparent, feedback from the coil 37 to the inductance36 will maintain oscillation, and the oscillator 24 will thereby developa definite ultrasonic frequency.

The ungrounded terminal of the transducer coil 11 is connected to thecollector 47 of transistor 26, and the emitter 48 of said transistor isconnected to the amplifier terminal 16. The base 49 of transistor 26 isconnected to one terminal of the transformer coil 38. The oppositeterminal of the transformer coil 38 is connected to the base 50 oftransistor 27.

The ungrounded terminal of the transducer coil 12 is connected to thecollector 51 of transistor 27, whereas the emitter 52 of transistor 27is connected to the amplifier input terminal 16. Coil 38 is centertapped at 53, and the center tap 53 is connected to the common emitterconnection of the transistors 26 and 27, namely the amplifier inputterminal 16. It will therefore be apparent that the emitter bases 49 and50 will be constantly in opposite phase relative to each other and willbe displaced 90 degrees in phase in opposite directions with respect tothe common connections of their emitters 48 and 52.

As is well known to those skilled in the art, transistors areessentially impedance devices controlling current flow rather thanvoltage. I have discovered that when a voltage is impressed on atransistor across the emitter and base, the resistance of the transistorfrom the emitter to the collector varies in accordance with saidimpressed voltage across the emitter and base. With one polarity of saidimpressed voltage the resistance of the transistor from the emitter tothe collector becomes very great. When the polarity is reversed, theresistance of the transistor from emitter to collector becomes verysmall. This enables the transistor to be used as a keying device, inaccordance with the equivalent circuit thereof illustrated in FIG. 10.For example, considering the transistor 26, a variable resistance isdefined between the emitter 48 and the collector 47, the magnitude ofwhich is controlled by the magnitude and polarity of the voltage appliedbetween the emitter 48 and the base 49. If the control voltagealternates, the transistor is alternately conducting and non-conductingin phase with the alternations of the control voltage applied across theemitter 48 and base 49.

Therefore, when transistor 26 conducts between its collector 47 andemitter 48, transistor 27 is non-conducting between its collector 51 andemitter 52, and vice versa. Therefore, the audio signals from thetransducer coils 11 and 12 are keyed at the frequency of the oscillator24 to provide the respective trains of pulses and 21 illustrated in FIG.9 and described above, said pulses being admitted simultaneously to theamplifier at its input terminal 16 and being superimposed so that theydefine a composite audio signal which may be conveyed and amplified byamplifier 13 in its ordinary manner.

The winding 39 of transformer 35 is center tapped at 53, and the centertap 53 is connected to the emitters 54 and 55 of the respective outputkeying transistors 28 and 29. One terminal of the coil 39 is connectedto the base 57 of transistor 28 and the opposite terminal thereof isconnected to the base 58 of the transistor 29. Collector 60, oftransistor 28 is connected to a terminal of loud speaker 30 andcollector 61 of transistor 29 is connected to a like terminal of loudspeaker 31. As above mentioned, the remaining terminals of the loudspeakers 30 and 31 are connected to the common terminal 33 of the outputtransformer 32 associated with the amplifier 13.

It will be seen that the transistors 28 and 29 are operated in the samemanner as the input keying transistors 26 and 27 and are operated inphase with said input keying transistors. The transistors 28 and 29therefore operate as keying devices, similar to the transistors 26 and27, with respect to the audio signal delivered by the secondary ofoutput transformer 32. The respective loud speakers 30 and 31 thusreceive the respective trains of pulses, similar to the pulses 20 and21, which are keyed at an ultrasonic frequency, the same as thatemployed with the transistors 26 and 27, whereby the loud speakers 30and 31 reproduce the original signals developed in the transducer coils11 and 12. Since the frequency of the oscillator 24 is above the audiblerange, this frequency is not audible in the output of the loud speakers30 and 31.

The input keying transistors 26 and 27 are preferably of the low noisetype, for example, may be of a type similar to the 2Nl90 type. Theoutput keying transistors 28 and 29 are preferably of the power outputtype, for example, may be of the 2N255 type.

FIGS. 3 to 8 illustrate examples of single-channel compatible stereosystems according to the present invention that may be used inbroadcasting, recording, or similar activities. Thus, F IG. 3diagrammatically illustrates a transmitting arrangement wherein therespective audio input signals are furnished through the keyers l8 and19 through respective band pass filters 70 and 71 which are designed topass frequencies in the audible range, for example, the frequenciesbetween 20 cycles and 19,750 cycles. The keyers l8 and 19 are operatedin opposite phase at an ultrasonic frequency which is furnished by afrequency multiplier 73. An oscillator 74 generates a subsonicfrequency, for example, a frequency of l3.75 cycles per second which isdelivered to the input of the multiplier 73. Multiplier 73 provides anoutput frequency which may be 1500 times that of the input frequencyfrom the oscillator 74, namely, 20.625 kilocycles. The output signalfrom the multiplier 73 is applied in opposite phase to the respectivekeyers 18 and 19 in the same manner as previously described inconnection with FIGS. 1 and 2 to key the input signals. The keyedsignals are simultaneously delivered to the input of an amplifier 13'.As will be presently pointed out with reference to FIG. 6, the subsonicfrequency of 13.75 cycles from the oscillator 74 and the ultrasonicfrequency of 20.625 kilocycles from the multiplier 73 are also appliedto the input of the amplifier 13. Thus, the output signal from theamplifier 13 contains the superposed keyed signals, amplified by theamplifier 13, and also contains the subsonic signal voltage of 13.75derived from the oscillator 74 and the ultrasonic keying signal of20.625 kilocycles furnished by the multiplier 73. Therefore, the outputfrom the amplifier 13', shown at 75, contains the composite keyed audiosignals, the subsonic signal of 13.75 cycles per second, and theultrasonic signal of 20.625 kilocycles per second.

As will be presently apparent, the reasons for including the subsonicsignal of 13.75 cycles in the output of the amplifier 13' is to providea means for developing a properly synchronized keying signal in thereproducing portion of the system in the event that the transmissionmedium between the transmission portion of the system and thereproducing portion of the system has poor high frequency response. Ifthis is the case, the ultrasonic keying signal of 20.625 kilocycles willbe substantially attenuated and will not be furnished to the reproducingportion of the system. However, the subsonic frequency of 13.75 cycleswill be present and may be multiplied in the reproducing portion of thesystem to develop the required ultrasonic keying signal. An arrangementfor accomplishing this result is diagrammatically illustrated in FIG. 5wherein the composite signal 75 is delivered to the input of anamplifier 76, the output of the amplifier being delivered throughrespective keyers 22 and 23 to loud speakers 30 and 31 similar to thoseillustrated in FIG. 2. The output from the amplifier 76 is also passedthrough a tuned filter 77 which is designed to pass only the subsonicfrequency of 13.75 cycles per second. Said subsonic frequency isfurnished to a conventional multiplier 73, similar to the conventionalmultiplier employed in FIG, 3, the multiplier providing an outputultrasonic signal voltage of 20.625 kilocycles which is applied inopposite phase to the respective keyers 22 and 23 to key the outputsignals delivered to the loud speakers 30 and 31 in synchronism with thekeying of the input signals furnished to the amplifier 13.

FIG. 4 diagrammatically illustrates a reproducing system wherein thetransmission medium has good high frequency response so that theultrasonic keying frequency contained in the transmitted signal at 75 isnot substantially attenuated. In the arrangement of FIG. 4, thetransmitted signal 75 is applied to the input of an amplifier 76, andthe output of the amplifier is applied through respective keyers 22 and23 to the respective loud speakers 30 and 31. The output from theamplifier 76 passes through a tuned filter 77' which is tuned to passthe ultrasonic keying frequency of 20.625 kilocycles. The keyingfrequency is then applied in opposite phase to the keyers 22 and 23 inthe same manner as shown in FIG. 2, whereby the signals applied to theloud speakers 30 and 31 are keyed in synchronism with the respectiveinput signals applied to the input of the amplifier 13'. The loudspeakers 30 and 31 will therefore substantially reproduce the audiosignals originally furnished to the system with the same stereophonicrelationship possessed by said audio signals.

FIG. 6 is a wiring diagram of the arrangement illustrated in FIG. 3,wherein the input keyers comprise respective transistors 26 and 27similar to those employed in the circuit of FIG. 2. The subsonicoscillator 74 is conventional in design and employs a transistor 34'.The oscillator output is available at a first output winding 80 of thetransformer 81, and is also available at a second output winding 82 ofsaid transformer. The signal available from the output winding 80 isapplied to one grid 83 of a pentagrid converter input tube 84 of theamplifier 13'. The signal available from the output coil 82 is appliedto the input of the conventional multiplier 73, the multiplier beingprovided with the output transformer 85 having the respective outputcoils 86 and 87. The output signal available from the coil 86 is appliedto another grid 88 of the amplifier input tube 84. The output coil 87 iscenter tapped at 89, said center tap being connected to one terminal 90of the primary of a transformer 91 employed to couple the audio inputsignal to the amplifier tube 84. One terminal of coil 87 is connected tothe base 49 of the keying transistor 26 and the opposite terminal ofsaid coil is connected to the base 50 of the keying transistor 27. Theemitters 48 and 52 of said transistors are connected to the terminal 90,whereby the control voltages applied between the emitters and the basesof the respective transistors are opposite in phase, providing thealternate keying of the signals in the manner above described. Thus, theaudio signals from the respective band pass filters 70 and 71 are keyedin opposite phase by the transistors 26 and 27, in the manner abovedescribed, and are applied as a composite signal to the terminal 90 oftransformer 91.

As above mentioned, the output from the multiplier 73 is an ultrasonicfrequency, which in the specific case above described is 20.625kilocycles. The composite signal containing the respective keyed audioinput signals is coupled to the input of the amplifier 13' by aconnection from the secondary terminal 92 of transformer 91 to anothergrid 93 of the amplifier tube 84. Therefore, the output from tube 84,shown at 75, contains the superimposed keyed audio input signals, thesubsonic frequency of 13.75 cycles per second, and the ultrasonic keyingfrequency of 20.625 kilocycles.

FIG. 7 is a wiring diagram of the arrangement illustrated schematicallyin FIG. 4. Thus, the output of the amplifier 76 is coupled to thereproducing circuits by an output transformer 94 having the secondarywindings 95 and 96. The secondary winding 95 is center tapped at 97 andthe center tap is connected to to respective emitters 54 and 55 of theoutput keying transistors 28 and 29, a suitable biasing battery 56 beingprovided in circuit between the emitters and the center tap 97 to renderthe transistors 28 and 29 normally conducting. One terminal of winding95 is connected to the base 57 of the transistor 28 and the oppositeterminal of said winding is connected to the base 58 of the transistor29. The collectors 60 and 61 of the respective transistors 28 and 29 areeach connected to one terminal of a respective loud speaker 30 and 31,as in FIG. 2. The remaining terminals of the loud speakers 30 and 31 areconnected to one terminal of the output winding 96. The remainingterminal of output winding 96 is connected to the emitters 54 and 55, asby a wire 98. Thus, the respective loud speakers 30 and 31 are connectedto the output winding 96 of output transformer 94 through the respectivecollectors and emitters of the transistors 28 and 29. The transistorsare keyed in opposite phase, as in FIG. 2, and are synchronized with theinput keying transistors 26 and 27 of FIG. 6. The filter 77' hasrespective tuned circuits 99 and 100 which are connected between theemitters 54, 55 and the respective bases 57 and 58 and which aredesigned to pass only the ultrasonic keying frequency of 20.625kilocycles.

FIG. 8 is a wiring diagram showing the arrangement schematicallyillustrated in FIG. 5. The output from the amplifier 76 is applied tothe primary winding of the output transformer 94'. Transformer 94' hasthe respective output coils 95' and 96. The output coil 96 is connectedto the respective loud speakers 30 and 31 through the emitters of therespective keying transistors 28 and 29, The connections of the loudspeakers 30 and 31 are the same as in FIG. 7. A suitable conventionalfilter 77 is connected across the output winding 95 of transformer 94'and is arranged to pass only the subsonic frequency of 13.75 cycles. Thewinding 95' is connected to the input of the multiplier 73, with thefilter 77 arranged to pass only said subsonic frequency. The subsonicfrequency of 13.75 cycles is multiplied in the multiplier 73 1,500 timesand is applied to the primary winding of a coupling transformer 101. Thecoupling transformer has a secondary winding 102 and is connectedthrough a suitable biasing battery 56 to the emitters 54 and 55 of thetransistors 28 and 29. One terminal of the output winding 102 isconnected to the base 57 of transistor 28 and the remaining terminal ofsaid winding is connected to the base 58 of the transistor 29. Thecollectors 60 and 61 of the transistors 28 and 29 are each connected toa terminal of a respective loud speaker 30 and 31, the remainingterminals of the loud speakers being connected to one terminal of theoutput winding 96, as by a wire 104. The remaining terminal of thewinding 96 is connected to the emitters 54 and 55, as 8 mentioned. Thebattery 56 is provided in order to render the transistors 28 and 29conducting when a normal monaural signal is received so that this signalwill be simultaneously reproduced by both speakers 30 and 31. However,the magnitude of the keying control voltages available from the outputwinding 102 is sufficient to cancel the voltage of the biasing battery56 and to render the transistors 28 and 29 alternately nonconducting.Thus, the output reproducing circuits containing the loud speakers 30and 31 are keyed in opposite phase in synchronism with the keying of theinput signals by the keying transistors 26 and 27 in FIG. 6. Since thesubsonic frequency of 13.75 cycles is available, the reproducing circuitof FIG. 8 is independent of the high frequency response of thetransmission medium.

it is possible to make a conventional stereophonic disc record which iscompatible with the system of the present invention and also with aconventional stereo system. Thus, the pulses of FIG. 9 may be recordedon one channel track of the stereo disc and the pulses 21 may berecorded on the other channel track. Then if the disc is played back inthe conventional stereo fashion, the respective stereophonically relatedaudio signals will be transmitted through the respective amplifiers tothe associated speakers of the system in the conventional manner.However, when the disc is played back into the system of the presentinvention, as in FIG. 2, for example, the two channels provide theoutputs consisting of the pulses 20 and the pulses 21, which aresuperimposed and which provide the composite signal at the wire 75 inFIGS. 3, 4 and 5. Of course, the composite signal should include thesub-multiple and ultra-sonic keying frequencies of FR}. 3.

The various methods of stereo transmission by radio may be similarlymade compatible with the system of the present present invention, bytransmitting superimposed pulse chains comprising the pulses 20 and thepulses 21 of FIG. 9. A conventional receiver which cannot separate thepulses will simultaneously reproduce the pulses from both channels, sothat applicants system is compatible with the procedures of conventionalradio broadcasting and reception.

While certain specific embodiments of an improved method and means ofutilizing a single transmission channel to convey a pair of audiosignals have been disclosed in the foregoing description, it will beunderstood that various modifications within the spirit of the inventionmay occur to those skilled in the art. Therefore, it is intended that nolimitations be placed on the invention except as defined by the scope ofthe appended claims.

I claim:

l. in a stereophonic transmission system having only two sources ofaudio signals: A first pair of transistors, each having an emitter, acollector and a base, a pair of audio signal sources, a signaltransmission channel, means connecting each source to the input of saidchannel through the collector and emitter of a respective transistor, anultrasonic sine wave alternating voltage generator, means connecting theoutput of said generator in opposite phase across the emitter and baseof the respective transistors, whereby to key the audio signals fromsaid sources in opposite phase at the frequency of said generator,deriving a composite signal comprising keyed components from said audiosignal sources at the input of the signal channel, a second pair oftransistors, each having an emitter, a collector and a base, a pair ofsound reproducing channels, means connecting the output of thetransmission channel to said reproducing channels through the collectorsand emitters of the respective second pair of transistors, meansinjecting a subsonic sub-multiple of the ultrasonic alternating voltageof said generator into said signal transmission channel, and means forderiving from said subsonic sub-multiple sine, continuous wavealternating voltages at the same frequency as said ultrasonicalternating voltage generator, and means for applying said sine wavealternating voltages in opposite phase across the emitters and bases ofthe second pair of transistors, whereby to separate the keyed audiosignal components.

2. in a stereophonic transmission system having only two sources ofaudio signals: a first pair of transistors, each having an emitter, acollector and a base, a pair of audio signal sources, a signaltransmission channel, means connecting each source to the input of saidchannel through the collector and emitter of a respective transistor, anultrasonic sine wave alternating voltage generator, means connecting theoutput of said generator in opposite phase across the emitter and baseof the respective transistors, whereby to key the audio signals fromsaid sources in opposite phase ultrasonic the frequency of saidgenerator, deriving a composite signal comprising keyed components fromsaid audio signal sources at the input of the signal channel, a secondpair of transistors, each having an emitter, a collector and a base, apair of sound reproducin channels, means in ecting a subsonicsub-multiple of the u trasonic alternating voltage of said generatorinto said signal transmission channel, means connecting the output ofsaid signal transmission channel to said reproducing channels throughthe collectors and the emitters of the respective second pair oftransistors, means to multiply said subsonic submultiple and to deriveoppositely phased ultrasonic continuous sine wave alternating voltagesof the same frequency as the output of said generator, and meansapplying said lastnamed alternating voltages across the emitters andbases of the second pair of transistors, whereby to key said reproducingchannels in opposite phase at said frequency and to separate the keyedaudio signal components.

1. In a stereophonic transmission system having only two sources ofaudio signals: A first pair of transistors, each having an emitter, acollector and a base, a pair of audio signal sources, a signaltransmission channel, means connecting each source to the input of saidchannel through the collector and emitter of a respective transistor, anultrasonic sine wave alternating voltage generator, means connecting theoutput of said generator in opposite phase across the emitter and baseof the respective transistors, whereby to key the audio signals fromsaid sources in opposite phase at the frequency of said generator,deriving a composite signal comprising keyed components from said audiosignal sources at the input of the signal channel, a second pair oftransistors, each having an emitter, a collector and a base, a pair ofsound reproducing channels, means connecting the output of thetransmission channel to said reproducing channels through the collectorsand emitters of the respective second pair of transistors, meansinjecting a subsonic sub-multiple of the ultrasonic alternating voltageof said generator into said signal transmission channel, and means forderiving from said subsonic sub-multiple sine, continuous wavealternating voltages at the same frequency as said ultrasonicalternating voltage generator, and means for applying said sine wavealternating voltages in opposite phase across the emitters and bases ofthe second pair of transistors, whereby to separate the keyed audiosignal components.
 2. In a stereophonic transmission system having onlytwo sources of audio signals: a first pair of transistors, each havingan emitter, a collector and a base, a pair of audio signal sources, asignal transmission channel, means connecting each source to the inputof said channel through the collector and emitter of a respectivetransistor, an ultrasonic sine wave alternating voltage generator, meansconnecting the output of said generator in opposite phase across theemitter and base of the respective transistors, whereby to key the audiosignals from said sources in opposite phase ultrasonic the frequency ofsaid generator, deriving a composite signal comprising keyed componentsfrom said audio signal sources at the input of the signal channel, asecond pair of transistors, each having an emitter, a collector and abase, a pair of sound reproducing channels, means injecting a subsonicsub-multiple of the ultrasonic alternating voltage of said generatorinto said signal transmission channel, means connecting the output ofsaid signal transmission channel to saId reproducing channels throughthe collectors and the emitters of the respective second pair oftransistors, means to multiply said subsonic submultiple and to deriveoppositely phased ultrasonic continuous sine wave alternating voltagesof the same frequency as the output of said generator, and meansapplying said last-named alternating voltages across the emitters andbases of the second pair of transistors, whereby to key said reproducingchannels in opposite phase at said frequency and to separate the keyedaudio signal components.